This invention relates in general to a dynamic shutter mechanism which as a variable shutter speed to control the exposure of a still or motion video camera. More particularly, this invention relates to a dynamic shutter mechanism having rotating shutter discs which are synched to both the horizontal and vertical sync frequencies of the video camera.
The need often arises for capturing high speed or rapidly changing scenes so that the scene is not blurred when it is played back in slow or stop motion. Where a standard video camera is used to record events, the frame or field rate of the camera is too slow to stop the motion of the event with a likelihood of blurring of the recorded image. Applications where the recording of high speed events may be desirable include athletic events, such as football, baseball, golfing or the like. Thus, it may be desirable to capture the motion of a horse in a horse race or to stop action in a football game in order to analyze the correctness of a referee's call on a play or to analyze the movements of a football player such as those of a quarterback in throwing a football to a receiver. In the case of a still video camera, shutter speeds greater than the frame or field rate of the video signal are also useful when it is desirable to increase the depth of field of a captured scene or when a slow telescopic lens is used with the camera.
Various rotating shutter mechanisms have been proposed for both film and video cameras with mixed results. If the shutter mechanism was manually adjustable, the possibility existed that a specific high speed event would not be captured because an inadequate shutter speed had been chosen by the camera operator. Mechanically adjustable shutter speed control of disc shutters used in motion picture film cameras are disclosed in U.S. Pats. Nos. 3,531,194; 3,602,585; 3,787,116; and 4,257,693. Manually adjustable shutter speed control for video cameras is disclosed in U.S. Pat. Nos. 4,161,003; 4,171,529 and 4,545,659. It should be apparent that many of the manually adjustable shutter speed control systems disclosed in the above patents are complex, bulky and ill suited for use in a lightweight, consumer type camera recorder.
Although automatic shutter speed control has been proposed for both motion picture film and video cameras, such automatic control systems have their limitations. Thus, in U.S. Pat. No. 3,490,835 an automatic film exposure system for motion picture cameras is disclosed in which a solenoid mechanism actuates a pair of spur gears which rotate the shutter discs. This arrangement is bulky and mechanically complex. Another automatic exposure control for motion picture cameras utilizing a pair of motors which independently rotate a pair of shutter discs is disclosed in U.S. Pat. No. 3,300,271. The two shutter discs are held together by a spring in order to intercouple the shutter blades and assure the same rotational speed thereof. Although this shutter mechanism is mechanically simple, it does not have the accuracy of speed control which is expected in modern video camera recorder systems.
An especially relevant disclosure to the present invention is that of U.S. Pat. No. 4,532,550 in which a pair of rotary shutters are driven by separate motors and locked to the vertical sync signal of the video signal by separate phase lock loops. This exposure control system is not as accurate as desired because speed control is sampled only once every 1/60 sec. and does not take into account speed variations in the two motors during the period between vertical sync signals.